Welcome to BYC! Glad you decided to join our flock. It is impossible to change the gender of an egg by temperature or any other method after it's laid. The chick's gender is determined at the time the embryo forms exactly the same as in mammals except that the female is the one who is heterozygous and determines gender instead of the male. The gender can't change later because the gene just isn't there. An egg either gets ZZ and is male, or ZW and is female. There is no way to spontaneously create a W or another Z. However it is potentially possible to alter what hatches. You can't change the gender in the eggs, but you can kill off more of one gender than the other. Your hatch rates would drop, and the dead eggs left behind would be mostly the same gender. Nothing is guaranteed to work though. The have been some inconclusive studies that indicate that cockerels might survive temperature extremes better than pullets. That means that making the temperature too hot or too cold will get you more cockerels than pullets by killing the pullets. Your unhatched eggs would be pullet eggs. Please feel free to ask any other questions you may have. We are here to help in any way we can. Good luck with your flock.